Coherent $ψ$(2S) photo-production in ultra-peripheral Pb-Pb collisions at $\sqrt{s}_{\rm NN}$ = 2.76 TeV

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2.76 TeV
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The ALICE Collaboration has performed the first measurement of the coherent y(2S) photoproduction
cross section in Pb-Pb collisions at the LHC. This charmonium excited state is
tagged via the y(2S) → l+l− and y(2S) → J/ψ π+π− decays, where the J/y decays into two
leptons. The analysis is based on an event sample corresponding to an integrated luminosity
of about 22 μb−1. The cross section for coherent y(2S) production in the rapidity interval -0.9
< y < 0.9 is ψ (2s) = 0.85+0.18-0.19 (sta +sys) mb. The obtained cross section and its ratio to
the J/ψ cross section are compared to those from theoretical models.